Abstract
AbstractThis study presents rare measurements and analysis of a nocturnal thunderstorm downburst on the 213 m tall Cabauw tower in The Netherlands. The event occurred on 12 March 2008 between 02:00 and 03:00 UTC and was measured using four ultrasonic 10-Hz anemometers positioned at 3, 60, 100, and 180 m above ground level. 1-second gusts in the outflow exceeded 30 m s−1 at 60 m and above. This wind event was accompanied by an abrupt change of wind direction from southwest to west. While the shift in wind direction corresponded with the change of upwind surface roughness, the time series of turbulence intensity and other turbulence characteristics were not affected. The statistical properties of this event were compared against the largest European database of thunderstorm winds measured in the Mediterranean. The study also demonstrated that primary and secondary vortex structures—secondary vortex being rarely observed in actual downbursts—developed at the forward edge of the cold outflow. The estimated diameter of the downdraft was 1200 m at 70 m above ground. The measured velocity profiles and friction velocity were compared against theoretical predictions of the Monin-Obukhov Similarity Theory (MOST). MOST without stratification adjustment overestimated measured friction velocity twofold. Alternative values for surface roughness during the outflow were derived based on the measured friction velocity and MOST-based fit of measured velocity profiles. Ceilometer and radar measurements were supplementary data in this analysis.
Highlights
Thunderstorm winds have been one of the main research topics in atmospheric sciences and wind engineering over the last several decades
While the surface pressure and temperature were only available as 10-min means, the pressure drop that preceded the temperature decline and precipitation is a known precursor of the approaching thunderstorm
This paper investigated a nocturnal downburst measured on a 213-m tall Cabauw tower in the Netherlands
Summary
Thunderstorm winds have been one of the main research topics in atmospheric sciences and wind engineering over the last several decades. Downbursts are a class of thunderstorm winds defined as strong downdrafts of cold air that originate in the cloud and spread out radially after impinging on the surface (Fujita and Byers 1977). The main contributors to downdraft development are the evaporation of hydrometeors inside and underneath the cloud, as well as the melting of ice and drag exerted by the falling hydrometeors. The increase of perturbation pressure with height provides a downward force on the air parcel. Due to their unpredictability, often intense magnitudes, highly three-dimensional flow field, and non-Gaussian velocity
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